The present invention relates to a method of an apparatus for the generation of random numbers. In particular, but not exclusively, the present invention relates to the use of a random pulse generator employing a radioactive source in the generation of random numbers.
Current methods for generating random numbers commonly employ mathematical algorithms using a fairly simple computer program and an 8-24 bit processor. The mathematical algorithms produce a stream of numbers which are calculated from an initial seed number using various possible mathematical formulae. Each random number generated is then used as the seed number for the next calculation. Such algorithms generate numbers which are generally described as pseudo-random because they are based on a calculation and so there is always a chance that a seed number will be repeated which would result in an infinite closed loop of repeated sequences of numbers.
Alternative known methods for generating random numbers employ natural unpredictable processes such as black body radiation, Brownian motion or radioactive decay. In JP 58-166448 the generation of long period pseudo-random numbers is described in which millicurie alpha radiation from radioactive uranium irradiates a 64,000 bit RAM. All of the bits of the RAM are initially set to xe2x80x980xe2x80x99 and each of the bits individually reset to xe2x80x981xe2x80x99 in the event an alpha particle is incident on the bit. After a predetermined period of time the number generated within the RAM is read as a pseudo-random number. With this method, pseudo-random numbers are generated by the spatial distribution of alpha particles incident on the RAM as a result of radioactive decay. However, the device described for generating the numbers is cumbersome, as it is necessary for the radioactive source to be positioned away from the RAM. Also, the device requires levels of radioactivity which for health reasons limit the situations in which the device could be used.
The present invention seeks to provide a method of and apparatus for the generation of random numbers which overcome at least some of the disadvantages with known methods described above. Moreover, where radioactive decay is used in the method the present invention provides apparatus which employs sufficiently low levels of radioactivity that the apparatus may be deemed to operate with xe2x80x98virtually no radioactivityxe2x80x99 (VNR).
The present invention provides a method of generating random numbers comprising detecting radioactive decay events during a predetermined time period, comparing the detected events to a selected output and generating a first signal where the detected events correspond to the selected output or generating a second signal where the detected events correspond to an output other than the selected output, wherein the time period is determined such that the probability of the detected events corresponding to the selected output is substantially equal to the probability of the detected events corresponding to an output other than the selected output.
Ideally, the time period is determined by measuring the mean number of radioactive decay events and calculating a time period for which the probability of the selected output being detected is xc2xd. In a preferred embodiment the steps of detecting radioactive decay events, comparing the detected events with a selected output and generating first or second signals are repeated cyclically to produce a string of first and second signals. In this way, as the predetermined time period is calculated so that the probability of the selected output being detected is equal to the probability of any output other than the selected output being detected, a truly random number can be generated from the string of first and second signals generated through repetition of the above mentioned steps.
Preferably, the first and second signals are binary bits and the series of bits are grouped to form binary numbers. The reverse binary number may also be determined from the binary bits and both the binary numbers and the reverse binary numbers may be converted into analogue numbers.
Additionally, the predetermined time period may be adjusted in dependence on the detected mean. The detected mean can be determined by summing repeated detected numbers of decay events to calculate a total output and then dividing the total output by the summed total of the time periods during which decay events were detected.
In an alternative method the individual outputs of a plurality of radioactive random event sources are detected during respective predetermined time periods and each detected output is compared to the selected output thereby generating a plurality of first or second signals simultaneously.
In a further aspect the present invention provides apparatus for generating random numbers comprising a decay detector for detecting radioactive decay events; a timer for controlling the time period during which decay events are detected by the decay detector; a comparator device for comparing the detected events with a selected output; and a number generator for generating a first signal when the detected events corresponds to the selected output and for generating a second signal when the detected events corresponds to an output other than the selected output.
Preferably, the decay detector includes a radioactive source and a detector in the form of a semiconductive substrate sensitive to the irradiation of decay products. For example the detector may be a PIN diode. Ideally, the apparatus is mounted on and forms part of a semiconductor chip.
Memory means may be provided for storing sequentially generated first and second signals thereby to form a random binary number and a digital-to-analogue converter may be provided for converting the stored random binary number into an analogue number.
Additionally, a feedback device may be provided for adjusting the predetermined time period in dependent on the detected mean output of the random event source.